Apparatus for titration and circulation of gases and circulation of an absorbent or adsorbent substance

ABSTRACT

The invention concerns an apparatus for the titration and circulation of gases to determine metal hydride storing properties, with improved response time, greater dynamic range in terms of the usable amount of powder and the maximum pressure accessible and increased sensitivity. The invention also concerns a circulating apparatus considerably reducing the time for analysing and determining the properties of absorbent and adsorbent materials during a large number of adsorption-desorption cycles. Both sets of apparatus are provided with a reference tube inside their oven, near the sample-holder. Said sample-holder tube and reference tube are connected on either side of the differential pressure sensor, thereby considerably enhancing the overall performance of the titration system.

FIELD OF THE INVENTION

The present invention relates to an improved apparatus for the titrationof gases, which can be used inter alia for the determination of thestorage properties of metal hydrides.

The invention also relates to an apparatus hereinafter called “cyclingapparatus”, which permits to evaluate the behaviour of a substance whenthis substance is subjected to a large number of gasabsorption/desorption cycles. This cycling apparatus can be used interalia for evaluating the degradation of storage properties of a metalhydride subjected to cycles of hydrogen absorption/desorption.

BRIEF DESCRIPTION OF THE PRIOR ART

There are presently apparatuses especially devised for the titration ofgases. These apparatuses are used in particular for determining thehydrogen absorption capacity and, therefore, the storage properties ofmetal hydrides. In the last case, they are particularly used for:

evaluating the storage capacity of metal hydrides as a function of theoperating pressure (pressure=f(H/M) where H is the number of hydrogenatoms and M is the number of metal atoms); and

evaluating the absorption and desorption kinetics (reaction dynamics) ofthe metal hydrides [H/M=f(time)].

FIG. 1 schematically illustrates the structure of an example of anexisting apparatus used for the titration of hydrogen. This apparatus isdisclosed in an article of Pascal TESSIER entitled “Hydrogen storage inmetastable Fe—Ti” of 1995.

As can be noticed, this existing apparatus comprises a main duct 1′which is connected by a valve V3′ to a source of hydrogen under pressure5′, and on which is mounted a pressure sensor (manometer) 7′ formeasuring the total pressure of hydrogen within the circuit.

The apparatus also comprises a first derivation duct 9′ which connectsthe main duct via a valve V6′ to a measuring chamber (13′) having theshape of a tube in which can be introduced a sample of the substance forwhich he absorption or desorption properties are to be measured. Thetube 13′ is located in a furnace 11′ having a temperature that can beadjusted at will as a function of the measurement to be carried out.

The apparatus further comprises a second derivation duct 15′ having afirst end 17′ connected to the main duct 1′ upstream of the connectionbetween the same and the first derivation duct 9′, and a second end 19′connected to the main duct downstream of the junction of the same withthe first derivation duct. This second derivation duct 15′ includes asmall tank 21′ of 50 cc and a differential pressure sensor 23′. A valveV11′ is located in the main duct 1′ between the junction 17′ and thefirst derivation duct 9′. Two other valves V5′ and V12′ are respectivelylocated on the second derivation duct 15′ between, on the one hand, thetank 21′ and the junction 17′ and, on the other hand, the differentialsensor 23′ and the junction 19′.

Last of all, the apparatus comprises a third derivation duct 27′connecting a pump 29′ via a valve VI′ to the main duct 1′ upstream ofthe junction 17′.

The valves mentioned hereinabove are operated by an informatized controlsystem 33′. The two sensor pressures 7′ and 23′ are also connected tothe control system. Most of the components of the apparatus areinsulated in an isothermal enclosure 35′ shown in dotted lines. A manualvalve V10′ is located in the derivation duct 9′. This manual valve V10′is kept permanently open except when the sample is inserted.

In use, after suitable calibration, one starts by creating a vacuumwithin the whole system by closing the valve V3′ and by opening all theother valves to connect all the ducts, the sample carrying tube 13′ andthe tank 21′ to the pump 29′. Then, all the valves are closed and themeasurement up is bean by adjusting the hydrogen source to a givenpressure. The valve V3′ is opened and then closed. Subsequently, thevalve V5′, V11′ and V12′ are opened in series. After a pause, the valveV5′ is closed and, after another pause, the valve V6′ is opened and themeasurement is carried out by measuring all the data given by bothpressure sensors 7′ and 23′.

This can be repeated several times with an increase in the hydrogenpressure, in order to obtain pressure/composition isotherm curves.

If the existing apparatuses for the titration of gases like the onedisclosed hereinabove are efficient, they are subject to very stringentlimitations in their use, because of their response time and thesaturation of their differential pressure sensors, which reduces thelimits of operation of the apparatus, its sensibility and the limits ofdetection of the same.

This problem is particularly important in that some metal hydrides likethe nanocrystalline alloys disclosed in the following recently laid-openpatent application Nos. CA-A-2,117,158 and WO-A-96/23906 naming theApplicant as one of the coowners, have very fast absorption anddesorption kinetics.

From a practical standpoint, it is possible to increase the operatingrange of the apparatus by modifying the sequences of opening of theadmission valves. However, the equilibrium time of the system is slower,which leads to a substantial lost of data at the beginning of eachmeasurement.

Accordingly, there is presently a real need for an apparatus for thetitration of gases where the response time would be improved and thedifferential pressure sensor would be less subject to saturation, withthe major drawback that such limits generate, namely a diminution of therange of use of the apparatus, expressed in amount of metal hydrideneeded for a given sensitivity threshold and maximum working pressure,both in PCT mode [pressure=f(H/M)] and in dynamic mode [(H/M=f(time)].

On the other hand, there are presently no apparatus available on themarket, at least to the knowledge of the Applicant, which would permitto carry out rapidly and in an efficient manner, titration measurementsat two different pressures and two different temperatures, in order tocharacterize a substance like an hydride, and more precisely, theefficiency of this hydride when it is subjected to a large number ofhydrogen absorption /desorption cycles.

It has already been proposed to use conventional titration apparatusesfor this purpose. However, because of the delays that are relativelylong for achieving furnace temperature equilibrium as well as thepressure equilibrium (a reequilibrium is required at reach cycle), theseapparatuses are poorly adapted for cycling, where it is necessary tochange the temperature of the furnace as well as the pressure quicklybetween each cycle during the course of measurements.

Therefore, there is also the need for a cycling apparatus which wouldpermit to carry out absorption/desorption cycles at two temperatures andtwo operating pressures in a fast, efficient and performing manner.

SUMMARY OF THE INVENTION

The present invention satisfies the two needs mentioned hereinabove byproviding:

on the one hand, a new apparatus for the titration of gases having animproved response time, a more important dynamics range relative to theamount of powder that is used and to the maximum operating pressure andan improved sensitivity; and

on the other hand, a cycling apparatus allowing a substantial reductionof the time required for the analysis and determination of theproperties of absorbing or desorbing materials during a large number ofabsorption/desorption cycles.

The apparatus according to the invention for the titration of a gascomprises:

a main duct (1) connected by a valve (V3) to a source of gas underpressure (5), said main duct being also connected to a first pressuresensor (7 a);

a first derivation duct (9) connecting the main duct (1) via a valve(V6) to a sample carrying tube (13) which is located in a furnace (11)of adjustable temperature and is devised to receive a sample of asubstance having gas absorption or adsorption/desorption properties tobe measured;

a second derivation duct (15) having ends (17,19) connected to the mainduct, at least one (19) of said ends being downstream of the firstderivation duct (9), said second derivation duct connecting in series avalve (V5), a tank (21) and a differential pressure sensor (23);

a third derivation duct (27) connecting a pump (29) via a valve (V1) tothe main duct (1);

an isothermal enclosure (35) for keeping the ducts and valves at astable and controlled temperature; and

a control system (33) for adjusting and controlling at will thetemperature of the furnace (11), the pressure of the gas and the valvesin real time.

This apparatus is characterized in that it further comprises:

a fourth derivation duct (37) connected via a valve (V7) to a referencetube (39) which has the same characteristics as the sample carrying tubeand is located together with the same in the furnace (11), said fourthderivation duct being connected to the second duct (15) between the tank(21) thereof and the differential pressure sensor (23).

As can be appreciated, the titration apparatus according to theinvention differs from the existing apparatuses in that it includes areference tube within the furnace close to the sample carrying tube. Thesample carrying tube and the reference tube are connected on both sidesof the differential pressure sensor, thereby leading to a substantialincrease in the general performances of the titration system.

Due to this structural difference, the titration apparatus according tothe invention has three major advantages.

First of all, its range of use is wider with respect to the amount ofpowder and the maximum pressure that can be used.

Secondly, the sensitivity of measurements is increased (the limit ofdetection is improved).

Thirdly, its response time is faster (larger dynamics range andreduction in the equilibrium time required for the differential pressuresensor).

On the other hand, the apparatus according to the invention for thecycling of a gas absorbing/desorbing material, is characterized in thatit comprises:

a furnace (111) with two compartments (171 a, 171 b) each having anadjustable temperature, said furnace being movable between two positionsby suitable means (175);

a main duct (101) connected by a valve (V103) to the source of gas to beabsorbed or adsorbed, this main duct being also connected to a pressuresensor (107);

a first derivation duct (109) connecting the main conduct (101) via avalve (V106) to a sample carrying tube (113) which is located within thefurnace (111) in such a manner as to be always located in one of thecompartments whatever be the position of the furnace, said samplecarrying tube being in one of the compartments when the furnace is inone of its two positions, and in the other compartment when the furnaceis in the other of its two positions;

two second derivation ducts (115 a and 115 b) independent from eachother and connectable alternatively to the main duct (101) via twocorresponding valves (V163 a and 163 b), each of said second derivationducts (115) including a valve (V105), a tank (121) and a differentialpressure sensor (123);

a third derivation duct (127) connecting a pump (129) via a valve (V101)to the main duct; and

two fourth derivation ducts (137 a and 137 b) each connecting one of thesecond derivation ducts via a valve (V107 a, V107 b) to a reference tube(139), said reference tubes (139 a and 139 b) of these fourth derivationducts being positioned within the furnace in such a manner as to be eachpositioned in one of the compartments of the furnace whatever be theposition of the same, one of the reference tubes being always associatedto the sample carrying tube whatever be the compartment in which thelatter is located.

As it can again be understood, the cycling apparatus according to theinvention comprises two furnaces and two enclosures that are kept underdifferent hydrogen pressures, and are connected to a simple yetefficient computerized interface. It permits to quickly carry outmeasurements under two different pressures and at two differenttemperatures, and therefore to evaluate the degradation of the storagecapacity of a substance like a metal hydride that is subjected toabsorption/desorption cycles.

As previously indicated, one of the main applications of these twoapparatuses is for evaluating in a more efficient and precise manner,the properties of recent hydrogen storage materials. This efficiency isdue to the fact that these apparatuses are particularly well adapted forthe measurement of very fast absorption/desorption kinetics.

However, it is worth mentioning that these apparatuses can also be usedfor numerous other applications, such as the absorption/desorption ofother gases, the absorption, for example, of natural gas, the evaluationof the problems of oxidation and reduction of materials, etc.

The invention and its advantages will be better understood upon readingthe following non-restrictive description of two preferred embodimentsof the invention given with reference to test results.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 identified as “prior art”, is a schematic representation of anexisting apparatus for the titration of gases;

FIG. 2 is a general view of an apparatus according to the invention forthe titration of gases;

FIG. 3 is a schematic representation of the apparatus for the titrationof gases shown in FIG. 2;

FIG. 4 is a PCT curve giving the value of the pressure measured withoutsample as a function of the ratio H/M measured and normalized when useis made of an existing apparatus as shown in FIG. 1 (▪) and when use ismade of an apparatus as shown in FIGS. 2 and 3 ();

FIG. 5a is a (dynamic) kinetics curve giving the value of the ratio H/Mmeasured without sample and normalised as a function of the time whenuse is made of an apparatus according to the invention;

FIG. 5b is a curve similar to the one of FIG. 5a, but obtained with anexisting apparatus, without a reference tube within the furnace;

FIG. 6 is a PCT curve [(pressure=f (H/M)] obtained with an apparatusaccording to the invention on a sample of 1.1696 g of LaNi₅ at atemperature of 30° C.;

FIG. 7 is a PCT curve obtained with an apparatus according to theinvention on a sample of 154.2 mg of Mg₂Ni at the temperature of 350°C.;

FIG. 8 is a dynamic curve [(H/M=f(time)] obtained with an apparatusaccording to the invention on a sample of 195 mg of a magnesium—basednanocrystalline material at a temperature of 300° C., the absorptionpressure being 200 psi (1380 kN/m²) and the pressure of desorption being0 psi (0 kN/m²);

FIG. 9 is a general view of a cycling apparatus according to theinvention;

FIG. 10 is a view showing the guiding rail of the furnace, the tworeference tanks and the sample carrying tube of the cycling apparatusshown in FIG. 9;

FIG. 11 is a schematic representation of the cycling apparatus shown inFIGS. 9 and 10;

FIG. 12 is a dynamic cycling curve measured with the apparatus shown inFIGS. 9 to 11 on a sample of 403.7 mg of LaNi₅, this curve giving thevalue of the ratio H/M as a function of the time under an absorptionpressure of 120 psi (830 kN/m²) and a desorption pressure of 35 psi (240kN/m²), both compartments of the furnace being kept at 50° C.;

FIG. 13 is a cycling curve measured with the apparatus shown in FIGS. 9to 11 on a sample of 403.7 mg of LaNi₅, this curve giving thetemperature of the sample as a function of the time under an absorptionpressure of 120 psi (830 kN/m²) and a desorption pressure of 35 psi (240kN/m²), both compartments of the furnace being kept at 50° C.;

FIG. 14 is a curve giving the value of the ratio H/M as a function ofthe time during the first and 350 cycles of absorption/desorption of asample of 411.5 mg of MgNi₅, the values being measured with theapparatus shown in FIGS. 9 to 11, under an absorption pressure of 270psi (1860 kN/m²) and a desorption pressure of 30 psi (205 kN/m²), bothcompartments of the furnace being kept at 335° C.

DESCRIPTION OF TWO PREFERRED EMBODIMENTS OF THE INVENTION

The apparatus according to the invention for the titration of gases asshown in FIGS. 2 and 3 comprises a main duct 1 connected by a valve V3to a source 5 of gas under pressure. In the preferred embodimenthereinafter exemplified, this gas is hydrogen. However, the apparatusaccording to the invention could be used with any other kind of gas.

The main duct 1 is directly connected to a first pressure sensor(manometer) 7 a capable of measuring pressures up to 1000 psia (6900kN/m²). It is also connected via a valve V4 to a second pressure sensor7 b that is more precise but capable of measuring pressure up to 250psia only (1700 kN/m²). It is worth mentioning that use will be made ofthe second sensor 7 b when the pressure of the gas injected into theapparatus is lower than 250 psia. If this pressure is higher than 250psia, the valve V4 will automatically close and only the sensor 7 a willmake the requested measurement of pressure.

The titration apparatus according to the invention also comprises afirst derivation duct 9 connecting the main duct 1 via a valve V6 to asample carrying tube 13 mounted in a detachable manner by means of atightness connection 12 and provided with an internal temperature probe(not shown). This tube 13 is intended to receive a sample, theproperties of the adsorption or absorption/desorption are to bemeasured. In use, this tube is located in a furnace 11 whose temperaturecan be adjusted and controlled at will. A manual valve V10 is locatedbetween the tightness connection 12 and the sample carrying tube 13.This valve is kept permanently opened, except when the tube 13 is beinghandled.

The titration apparatus according to the invention further comprises asecond derivation duct 15. This duct has a first end 17 connected to themain duct substantially at the same level as the first derivation duct9, and a second end 19 connected to the main duct at the same level orupstream of the pressure sensors 7 a and 7 b. This second derivationduct 15 includes a small tank 21 of 50 cc, and a differential pressuresensor 23.

The titration apparatus according to the invention still comprises athird derivation duct 27 for connecting a vacuum pump 29 via V1 to theduct 1, upstream of the junction 17.

Except for three minor differences, namely the absence of the valvesV11′ and V12″ shown in FIG. 1 and the use of two pressure sensors 7 aand 7 b instead of a single one 7′, the basic structure of the apparatusaccording to the invention is, so far, identical to the one of anyexisting titration apparatus.

The main structural feature distinguishing the invention over the stateof the art lies in the presence of a fourth derivation duct 37. Thisduct connects the second derivation duct 15 via a valve V7 to a tubehereinafter called “reference tube” 39, which is located within thefurnace 11 close to the sample-carrying tube 13. The reference tube hasthe same characteristics (structure, volume, . . . ) as thesample-carrying tube.

The junction 41 between the fourth derivation duct 37 and the secondduct 15 is located between the tank 21 and the differential pressuresensor 23.

A fifth derivation duct 49 is connected to the main duct 1 upstream ofthe junctions of the same with the first and second derivation ducts 9and 15. This fifth duct connects a large desorption tank 51 of 1 litervia a valve V9 to the main duct.

A sixth and last derivation duct 43 is connected to the main duct,substantially at the level of the junction of the same with the thirdderivation duct 27. This sixth duct 43 connects the main duct 1 via avalve V2 to a high pressure exit 45 and to a source 47 of inert gasunder pressure, such as nitrogen, argon or helium. A valve V8 is locatedbetween this gas source 47 and the duct 43.

A small buffer tank 53 of 50 cc is advantageously located on the mainduct between the junctions 17 and 19. This buffer tank 53 is used tocompensate the differences in volume in the circuit resulting from thelengthy of each duct. Depending on the compensation to be carried out,which is easily determined during the calibration of the apparatus, thebuffer tank can be filled with metal chunks to reduce its dead volumeand, accordingly, adjust its residual volume to the difference of volumeto be compensated within the ducts.

Like in the case of the existing apparatuses, most of the components ofthe apparatus according to the invention are kept insulated in anisothermal enclosure 35. The valves, the hydrogen and nitrogen sources,the pump, the high pressure exit and the temperature of the furnaces arecontrolled in real time by a computerized system 33 that is easilyprogrammable.

The apparatus according to the invention for the titration of gases thathas just been described, operates as follows.

After assembling the circuit, one proceeds to a calibration in order todetermine the dead volume in the ducts between the valves and ensurethat there is a same volume on both sides of the differential manometerduring the measurements (as will be better understood hereinafter).During this first step, the buffer tank 53 is filled if necessary withchunks of iron or any other metal that does not absorb hydrogen. Afterhaving inserted the sample of metal hydride to be tested in thesample-carrying tube 13 and having closed valves V2 and V3 of the fifthand sixth derivation ducts, one may proceed to a purge of all the ducts.Then, the pump 29 is turn on and the valve V1 is opened. Thereafter,some other valves of the first, second and fourth derivation ducts 9, 15and 37 are opened.

Then, the absorption measurements may be started. To do so, one may setthe requested absorption pressures and temperature of the furnace 11.Then, the valves V6 and V7 leading to the sample-carrying tube 13 and tothe reference tube 39 are closed while both of them are still undervacuum.

The valve V3 is opened and re-closed to connect all the ducts to thehydrogen source 5 and to place them under the requested absorptionpressure. Once this is done, the valve V5 is closed and the valves V6and V7 are simultaneously opened. Such creates a release of gas towardsthe tubes 13 and 39. Then, one may proceed to the simultaneousmeasurements of pressure by means of the sensors 7 a and/or 7 b and 23.

Once the measurements are completed, the high pressure exit 45 isoperated and the valve V2 is opened after having closed the valves V6and V7 in order to remove the hydrogen from the apparatus. Prior toopening the valve V2, one may open the valve V8 in order to mix nitrogenfrom the source 47 with the hydrogen that is evacuated from theapparatus. This improves the safety of the apparatus in use, by reducingthe risk of fire.

Once the purge is completed, the desorption measurement can then becarried out.

To do so, the valve V9 of the fifth derivation duct is opened in orderto connect the large tank 51 to the main duct. The extra volume providedby the tank “improves” the desorption. In this connection, one mayunderstand that the average volume of the ducts and tubes on both sidesof the differential sensor is of about 100 cc. By adding a volume of1000 cc within the circuit, the dead volume for receiving the desorbedhydrogen is multiplied by 10. This increases the control of pressureincrements.

After having adjusted the pressure in the main duct and closed the valveV5, one may then reopen again the valves V6 and V7 and carry out therequired measurements with the pressure sensors 7 a and/or 7 b and 23.

Once everything is completed, the valve V6 can be closed again and onemay either change the sample or starts another absorption and/ordesorption measurement at different pressure and/or temperature.

As mentioned hereinabove, the apparatus for the titration of gasesaccording to the invention, thanks to the presence of its reference tubeclose to the sample-carrying tube located within the furnace, hasnumerous advantages as compared to the existing apparatuses. Amongstthese advantages, one may mention:

a wider range of operation in terms of amount of powder and maximumpressure that can be achieved;

an increased sensitivity; and

an improved response time (wide dynamic range).

This apparatus is of simple use. In fact, its use is advantageouslysimplified and rendered more efficient and convivial thanks to the useof a computerized interface incorporated into its control system. Thiscontrol software will not be described and claimed hereinabove.

Numerous tests were carried out on several prototypes at the Institut deRecherche d'Hydro-Québec (IREQ). Some of the results obtained duringthese tests are shown in FIGS. 4 to 8.

FIGS. 4 and 5 give PCT measurements [pressure=f(H/M)] and dynamicmeasurements carried out with an apparatus operating under vacuum(without sample) when this apparatus is provided with a reference tube(apparatus according to the invention) and when it does not have such atube (existing apparatus).

As can be seen on FIG. 4, there is a substantial decrease in thevariations of the H/M value with an empty cell when use is made of areference tube (this improvement is of one order in magnitude). Also, ascan be seen when comparing FIGS. 5a with 5 b, there is also a verysubstantial decrease in the equilibrium time of the H/M value when useis made of a reference tube (this equilibrium time is reduced from morethan 400 seconds without a reference tube to about 1 second with areference tube). This last decrease is essential for the measurements ofabsorption/desorption carried out on metal hydrides that are veryefficient, such as the nanocrystalline hydrides disclosed in theCanadian and international patent applications mentioned hereinabove,which have very fast absorption kinetics.

FIGS. 6 and 8 are illustrative of the quality of the results that can beobtained with different types of samples of different weight atdifferent temperatures.

The cycling apparatus according to the invention as shown in FIGS. 9, 10and 11 derives directly from the titration apparatus that has just beendescribed, except that it is devised to evaluate the characteristics ofa gas-absorbing or -adsorbing material, and more precisely of a metalhydride, in order to determine the degradation of its storing propertieswhen this material is subjected to a great number ofabsorption/desorption cycles. Once again, one will understand that theinvention is not restricted to metal hydrides and that the same cyclingapparatus could be used for evaluating the absorption capacities of anyother kind of substance.

As it is better shown in FIG. 11, the cycling apparatus according to theinvention has a basic structure very similar to the one of the titrationapparatus previously disclosed. For this reason, all the identicalstructural components have been identified in the same way with the samereference numerals, to which has been added the number 100.

Thus, the cycling apparatus comprises a main duct 101 connected to ahydrogen source 105 via a valve V103. This main duct includes a pressuresensor 107.

A first derivation duct 109 connects the main duct 101 via valves V106and V10 to a sample-carrying tube 113 that is mounted in a detachablemanner by means of a tightening connection 112 and is provided with aninternal temperature sensor 114 (see FIG. 10).

Two second derivation ducts 115 a and 115 b are connected to twobranches 161 a and 161 b of the main duct, upstream the sensor pressure107. These branches are respectively provided with valves V163 a andV163 b. These second ducts and their connections will be describedhereinafter in greater detail.

A third derivation duct 127 connects the main duct to a vacuum pump 129.

Two fourth derivation ducts 137 a and 137 b including two valves V107 aand V107 b, respectively connect the second ducts 115 a and 115 b to tworeference tubes 139 a and 139 b consisting of small tanks of 50 cc thatare respectively positioned on top and below the sample-carrying tube113.

Last of all, a further derivation duct 143 connects the main duct 101 toan exit device 145 via a valve V102. A nitrogen source 147 is connectedvia a valve V108 to this last duct 143 to mix the nitrogen with the gasexiting from the duct 143 and thus to reduce the risk of fire when thisgas is inflammable.

Two large tanks 151 a and 151 b each with a dead volume of 2.25 liters,are respectively connected by valves V109 a and V109 b to the secondducts 115 a and 115 b. One may understand that the volume of these tanksmay of course be modified at will.

Valve 105 a and 105 b and differential pressure sensors 123 a and 123 bare mounted in series on the second ducts 115 a and 115 b.

Last of all, two small tanks 121 a and 121 b of 50 cc each arerespectively connected to the second derivation ducts 115 a and 115 bbetween the valves V105 a and V105 b and the differential pressuresensors 123 a and 123 b.

As can now be noticed, each of the second derivation conducts 115 a and115 b to which a reference tube 139 a or 139 b, is associated isidentical in terms of structure and operation to the circuit formed bythe second, fourth and fifth derivation ducts 15, 37 and 49 of thetitration apparatus previously described. The only difference lies inthat these second ducts 115 a and 115 b are connectable in analternative manner directly to the main duct 101 by means of a valveV161 a and V161 b, this being of course essential to obtain therequested cycling.

One will therefore understand that the sequence of operation isabsolutely identical to the one that has already been described indetail, except that, when the reference tube 139 a is used for ameasurement, the valve V163 b is closed for isolating the second duct115 b and all the elements associated to the same, whereas, when thereference tube 139 b is used, then the valve V163 a is closed.

Most of the components disclosed hereinabove are located in anisothermal enclosure 135 and connected to a control system 133.

To induce the variation of temperature during cycling, the furnace 111in which the sample-carrying tube 113 and the reference tubes 139 a and139 b are located, may have two compartments 171 a and 171 b that arecoaxial. The temperature of each compartment can be adjusted andcontrolled independently from the other. These two compartments aremounted onto a jack 173 connected to a source of compressed air 175 andactionable at a distance by means of the same. For each cycle, the jack173 moves together both compartments of the furnace 111 upwardly ordownwardly, as shown by the arrow A. In lower position, which is the oneillustrated in FIG. 11, the sample-carrying tube 113 and the referencetube 139 a are both in the compartment 171 a. Then, the requestedabsorption/desorption is carried out inside this compartment 171 a atthe temperature of the same, by opening the second duct 115 a. In upperposition, the sample carrying tube 113 is with the reference tube 139 bin the compartment 171 b. Then, the requested desorption/absorption iscarried out at the temperature of the compartment 171 b, by opening thesecond conduct 115 b.

As can therefore be appreciated, the cycling apparatus according to theinvention uses a furnace with two compartments as well as two pressureenclosures for the absorption and desorption. Such permits tosubstantially reduces the time required for evaluating the cyclingproperties of absorbing materials such as metal hydrides.

Tests were carried out on the prototype of a cycling apparatus aspreviously described. Some of the obtained results are reported in FIGS.12 to 14.

FIG. 12 is a dynamic cycling curve [H/M=f(time)] obtained with a sampleof LaNi₅. This curve illustrates quite well the repetition of cycles. Inthis case, both compartments of the furnace were maintained at the sametemperature of 50° C.

FIG. 13 is a cycling curve giving the value of the temperature of asample of LaNi₅ as a function of the time. Once again, both compartmentswere kept at the same temperature of 50° C. This curve shows that thetemperature of the sample varies in a substantial manner during theabsorption/desorption cycles.

Last of all, FIG. 14 is a dynamic curve showing the difference ofbehaviour of a sample of Mg₂Ni between the first and 305th cycles ofabsorption/desorption. As can be seen, the material looses about 15% ofits hydrogenation capacity and is subjected to a substantial degradationof its desorption kinetics in use.

Of course, numerous modifications could be made to the embodiments thatjust have been described without departing the scope of the presentinvention as defined in the appended claims. Thus, one may understandthat use could easily be made of temperatures of higher or lower thanthose described in this document. Use could also be made of, forexample, a refrigerator for absorption at low temperature, while thedesorption at high temperature could always be carried out in a furnace.Therefore, there is possibility of using other alternative methods ofdesorption or absorption.

What is claimed is:
 1. An apparatus for the titration of a gas, of thetype comprising: a main duct connected by a first valve to a source ofgas under pressure, said main duct being also connected to a firstpressure sensor; a first derivation duct connecting the main duct via asecond valve to a sample carrying tube which is located in a furnace ofadjustable temperature and is devised to receive a sample of a substancehaving gas absorption or adsorption/desorption properties to bemeasured; a second derivation duct having first and second endsconnected to the main duct, said second derivation duct connecting inseries a third valve, a first tank and a differential pressure sensor; athird derivation duct connecting a pump via a fourth valve to the mainduct; an isothermal enclosure for keeping the ducts and valves at astable and controlled temperature; and a control system for adjustingand controlling at will the temperature of the furnace, the pressure ofthe gas and the valves in real time; said apparatus being characterizedin that it further comprises: a fourth derivation duct connected via afifth valve to a reference tube which has the same characteristics asthe sample carrying tube and is located together with the same in thefurnace, said fourth derivation duct being connected to the second ductbetween the first tank thereof and the differential pressure sensor; anda buffer tank mounted onto the main duct between the first and secondends of the second derivation duct, said buffer tank being filled upwith metal chunks to reduce its dead volume, and used for compensatingany difference in volume in the main and derivation ducts.
 2. Anapparatus as defined in claim 1, characterized in that it furthercomprises: a fifth derivation duct connected to the main duct upstreamthe first and second derivation ducts, this fifth derivation duct beingconnected to a second tank via a sixth valve, said second tank beingused for the desorption by increasing substantially the dead space ofthe duct when the sixth valve is open.
 3. An apparatus as defined inclaim 2, characterized in that it further comprises: a sixth derivationduct connected to the main duct, said sixth derivation duct beingconnected to a gas outlet via a seventh valve.
 4. An apparatus asdefined in claim 3, characterized in that it further comprises: a sourceof inert gas connected via an eighth valve to the sixth derivation ductfor mixing the inert gas with the gas exiting through this sixthderivation duct and thus reducing the risk of fire when this gas isinflammable.
 5. An apparatus as defined in claim 1, characterized inthat it further comprises: a second pressure sensor connected to themain duct via a ninth valve, this second sensor being adapted to measurepressures up to a maximum pressure which is lower than the maximumpressure which can be measured by said first pressure sensor.
 6. Aprocess for evaluating the hydrogen storage capacity of a metal hydrideas a function of the operating pressure (pressure=f(H/M), where H is thenumber of hydrogen atoms and M is the number of metal atoms), whichcomprises: calibrating an apparatus as claimed in claim 1, in order todetermine the dead volume in the ducts between the valves; inserting asample of said metal hydride to be tested in the sample carrying tube,closing the first valve and proceeding to a purge of all the ducts;turning on the pump and opening the valves of the first, second andfourth derivation ducts; setting the requested absorption pressure andtemperature of the furnace and closing the second and the fifth valvesleading to the sample, the carrying tube and the reference tube whileboth of said tubes and are still under vacuum; opening the first valveand reclosing it to connect all the ducts to the source of gas, said gasbeing hydrogen, and thus to place all of ducts under the requestedabsorption pressure; closing the third valve and simultaneously openingthe second and the fifth valves to create a release of gas towards thecarrying tube and the reference tube proceeding to a simultaneousmeasurement of pressure by means of the first pressure sensor and thedifferential pressure sensor; closing the second and the fifth valvesand opening the seventh valve to remove hydrogen from the apparatus;adjusting the pressure in the main duct, closing the fifth valve andreopening again the second and the fifth valves; proceeding to anothermeasurement of pressure by means of the first pressure sensor and thedifferential pressure sensor; and closing the second valve prior toreusing the apparatus.
 7. A process for evaluating the hydrogenabsorption and desorption kinetics of a metal hydride (H/M=f(time),where H is the number of hydrogen atoms and M is the number of metalatoms), which comprises calibrating an apparatus as claimed in claim 1,in order to determine the dead volume in the ducts between the valves;inserting a sample of said metal hydride to be tested in the samplecarrying tube, closing the first valve and proceeding to a purge of allthe ducts; turning on the pump and opening the valves of the first,second and fourth derivation ducts; selecting one of two furnacepositions and setting the requested absorption pressure and temperatureof the furnace and closing the second and the sixth valves leading tothe sample, the sample carrying tube and the reference tube while all ofsaid tubes are still under vacuum; opening the first valve and reclosingit to connect all the ducts to the source of gas, said gas beinghydrogen, and thus to place all of the ducts under the requestedabsorption pressure; closing the third valve and simultaneously openingthe second and the sixth valves to create a release of gas towards thesample carrying tube and the reference tubes; and proceeding to asimultaneous measurement of time and of pressure by the pressure sensor.8. A cycling apparatus for evaluating the behaviour of a gas absorbingof adsorbing substance when this substance is subjected to a largenumber of absorption/desorption cycles, characterized in that itcomprises: a furnace with two compartments each having an adjustabletemperature, said furnace being movable between two positions bysuitable means; a main duct connected by a first valve to a source ofgas to be absorbed or adsorbed, this main duct being also connected to apressure sensor; a first derivation duct connecting the main duct via asecond valve to a sample carrying tube which is located within thefurnace in such a manner as to be always positioned in one of thecompartments whatever be the position of the furnace, said samplecarrying tube being in one of the compartments when the furnace is inone of its two positions, and in the other compartment when the furnaceis in the other of its two positions; two second derivation ductsindependent from each other, each of said second derivation ductsconnectable alternatively to the main duct via a corresponding thirdvalve and connecting in sequence an inlet, a fourth valve, a first tank,a differential pressure sensor and an outlet; a third derivation ductconnecting a pump via a fifth valve to the main duct; and two fourthderivation ducts each connecting the outlet of one of the secondderivation ducts via a sixth valve to a reference tube, said referencetubes of these fourth derivation ducts being positioned within thefurnace in such a manner as to be each positioned in one of thecompartments of the furnace whatever be the position of the same, one ofthe reference tubes being always associated to the sample carrying tubewhatever be the compartment in which the latter is located.
 9. Anapparatus as claimed in claim 8, characterized in that it furthercomprises: two second tanks respectively connected via a seventh valveto the second derivation ducts, these second tanks helping thedesorption by substantially increasing the dead space of the ducts whenthe seventh valve are opened.
 10. An apparatus as claimed in claim 9,characterized in that it further comprises: a fifth derivation ductconnected to the main duct said other derivation duct being connected toa gas exit via an eighth valve.
 11. An apparatus as defined in claim 10,characterized in that it further comprises: a source of inert gasconnected via a ninth valve to the fifth derivation duct for mixing theinert gas with the gas exiting through the fifth derivation duct andthus for reducing the risk of fire when this gas is inflammable.
 12. Anapparatus as defined in claim 8, characterized in that it furthercomprises: an internal temperature sensor located within the samplecarrying tube.
 13. A process for evaluating the hydrogen adsorption anddesorption kinetics of a metal hydride as a function of the time (H/M=f(time) where H is the number of hydrogen atoms and M is the number ofmetal atoms), which comprises: calibrating an apparatus as claimed inclaim 8, in order to determine the dead volume in the ducts between thevalves; inserting a sample of said metal hydride to be tested in thesample carrying tube, closing the first valve and proceeding to a purgeof all the ducts; turning on the pump and opening the valves of thefirst, second and fourth derivation ducts; setting the requestedabsorption pressure and temperature of the furnace and closing thesecond and the fifth valves leading to the sample, the carrying tube andthe reference tube while both of said tubes and are still under vacuum;opening the first valve and reclosing it to connect all the ducts to thesource of gas, said gas being hydrogen, and thus to place all of ductsunder the requested absorption pressure; closing the third valve andsimultaneously opening the second and the fifth valves to create arelease of gas towards the carrying tube and the reference tubeproceeding to a simultaneous measurement of pressure by means of thefirst pressure sensor and the differential pressure sensor; closing thesecond and the fifth valves and opening the seventh valve to removehydrogen from the apparatus; adjusting the pressure in the main duct,closing the fifth valve and reopening again the second and the fifthvalves; proceeding to another measurement of pressure by means of thefirst pressure sensor and the differential pressure sensor; and closingthe second valve prior to reusing the apparatus.